Kafka Wire Protocol

This document covers the wire protocol implemented in Kafka. It is meant to give a readable guide to the protocol that covers the available requests, their binary format, and the proper way to make use of them to implement a client. This document assumes you understand the basic design and terminology described here

• Preliminaries
  • Network
  • Partitioning and bootstrapping
  • Partitioning Strategies
  • Batching
  • Versioning and Compatibility
• The Protocol
  • Protocol Primitive Types
  • Notes on reading the request format grammars
  • Common Request and Response Structure
  • Message Sets
• Constants
  • Error Codes
  • Api Keys
• The Messages
• Some Common Philosophical Questions

Preliminaries

Network

Kafka uses a binary protocol over TCP. The protocol defines all apis as request response message pairs. All messages are size delimited and are made up of the following primitive types.

The client initiates a socket connection and then writes a sequence of request messages and reads back the corresponding response message. No handshake is required on connection or disconnection. TCP is happier if you maintain persistent connections used for many requests to amortize the cost of the TCP handshake, but beyond this penalty connecting is pretty cheap.

The client will likely need to maintain a connection to multiple brokers, as data is partitioned and the clients will need to talk to the server that has their data. However it should not generally be necessary to maintain multiple connections to a single broker from a single client instance (i.e. connection pooling).

The server guarantees that on a single TCP connection, requests will be processed in the order they are sent and responses will return in that order as well. The broker's request processing allows only a single in-flight request per connection in order to guarantee this ordering. Note that clients can (and ideally should) use non-blocking IO to implement request pipelining and achieve higher throughput. i.e., clients can send requests even while awaiting responses for preceding requests since the outstanding requests will be buffered in the underlying OS socket buffer. All requests are initiated by the client, and result in a corresponding response message from the server except where noted.

The server has a configurable maximum limit on request size and any request that exceeds this limit will result in the socket being disconnected.

Partitioning and bootstrapping

Kafka is a partitioned system so not all servers have the complete data set. Instead recall that topics are split into a pre-defined number of partitions, P, and each partition is replicated with some replication factor, N. Topic partitions themselves are just ordered "commit logs" numbered 0, 1, ..., P.

All systems of this nature have the question of how a particular piece of data is assigned to a particular partition. Kafka clients directly control this assignment, the brokers themselves enforce no particular semantics of which messages should be published to a particular partition. Rather, to publish messages the client directly addresses messages to a particular partition, and when fetching messages, fetches from a particular partition. If two clients want to use the same partitioning scheme they must use the same method to compute the mapping of key to partition.

These requests to publish or fetch data must be sent to the broker that is currently acting as the leader for a given partition. This condition is enforced by the broker, so a request for a particular partition to the wrong broker will result in an the NotLeaderForPartition error code (described below).

How can the client find out which topics exist, what partitions they have, and which brokers currently host those partitions so that it can direct its requests to the right hosts? This information is dynamic, so you can't just configure each client with some static mapping file. Instead all Kafka brokers can answer a metadata request that describes the current state of the cluster: what topics there are, which partitions those topics have, which broker is the leader for those partitions, and the host and port information for these brokers.

In other words, the client needs to somehow find one broker and that broker will tell the client about all the other brokers that exist and what partitions they host. This first broker may itself go down so the best practice for a client implementation is to take a list of two or three urls to bootstrap from. The user can then choose to use a load balancer or just statically configure two or three of their kafka hosts in the clients.

The client does not need to keep polling to see if the cluster has changed; it can fetch metadata once when it is instantiated cache that metadata until it receives an error indicating that the metadata is out of date. This error can come in two forms: (1) a socket error indicating the client cannot communicate with a particular broker, (2) an error code in the response to a request indicating that this broker no longer hosts the partition for which data was requested.

1. Cycle through a list of "bootstrap" kafka urls until we find one we can connect to. Fetch cluster metadata.
2. Process fetch or produce requests, directing them to the appropriate broker based on the topic/partitions they send to or fetch from.
3. If we get an appropriate error, refresh the metadata and try again.

Partitioning Strategies

As mentioned above the assignment of messages to partitions is something the producing client controls. That said, how should this functionality be exposed to the end-user?

Partitioning really serves two purposes in Kafka:

1. It balances data and request load over brokers
2. It serves as a way to divvy up processing among consumer processes while allowing local state and preserving order within the partition. We call this semantic partitioning.

For a given use case you may care about only one of these or both.

To accomplish simple load balancing a simple approach would be for the client to just round robin requests over all brokers. Another alternative, in an environment where there are many more producers than brokers, would be to have each client chose a single partition at random and publish to that. This later strategy will result in far fewer TCP connections.

Semantic partitioning means using some key in the message to assign messages to partitions. For example if you were processing a click message stream you might want to partition the stream by the user id so that all data for a particular user would go to a single consumer. To accomplish this the client can take a key associated with the message and use some hash of this key to choose the partition to which to deliver the message.

Batching

Our apis encourage batching small things together for efficiency. We have found this is a very significant performance win. Both our API to send messages and our API to fetch messages always work with a sequence of messages not a single message to encourage this. A clever client can make use of this and support an "asynchronous" mode in which it batches together messages sent individually and sends them in larger clumps. We go even further with this and allow the batching across multiple topics and partitions, so a produce request may contain data to append to many partitions and a fetch request may pull data from many partitions all at once.

The client implementer can choose to ignore this and send everything one at a time if they like.

Versioning and Compatibility

The protocol is designed to enable incremental evolution in a backward compatible fashion. Our versioning is on a per-api basis, each version consisting of a request and response pair. Each request contains an API key that identifies the API being invoked and a version number that indicates the format of the request and the expected format of the response.

The intention is that clients would implement a particular version of the protocol, and indicate this version in their requests. Our goal is primarily to allow API evolution in an environment where downtime is not allowed and clients and servers cannot all be changed at once.

The server will reject requests with a version it does not support, and will always respond to the client with exactly the protocol format it expects based on the version it included in its request. The intended upgrade path is that new features would first be rolled out on the server (with the older clients not making use of them) and then as newer clients are deployed these new features would gradually be taken advantage of.

Currently all versions are baselined at 0, as we evolve these APIs we will indicate the format for each version individually.

The Protocol

Protocol Primitive Types

The protocol is built out of the following primitive types.

Fixed Width Primitives

int8, int16, int32, int64 - Signed integers with the given precision (in bits) stored in big endian order.

Variable Length Primitives

bytes, string - These types consist of a signed integer giving a length N followed by N bytes of content. A length of -1 indicates null. string uses an int16 for its size, and bytes uses an int32.

Arrays

This is a notation for handling repeated structures. These will always be encoded as an int32 size containing the length N followed by N repetitions of the structure which can itself be made up of other primitive types. In the BNF grammars below we will show an array of a structure foo as [foo].

Notes on reading the request format grammars

The BNFs below give an exact context free grammar for the request and response binary format. The BNF is intentionally not compact in order to give human-readable name. As always in a BNF a sequence of productions indicates concatenation. When there are multiple possible productions these are separated with '|' and may be enclosed in parenthesis for grouping. The top-level definition is always given first and subsequent sub-parts are indented.

Common Request and Response Structure

All requests and responses originate from the following grammar which will be incrementally describe through the rest of this document:

RequestOrResponse => Size (RequestMessage | ResponseMessage)
Size => int32

Message Sets

A description of the message set format can be found here. (KAFKA-3368)

Constants

Error Codes

We use numeric codes to indicate what problem occurred on the server. These can be translated by the client into exceptions or whatever the appropriate error handling mechanism in the client language. Here is a table of the error codes currently in use:

Api Keys

The following are the numeric codes that the ApiKey in the request can take for each of the below request types.

The Messages

This section gives details on each of the individual API Messages, their usage, their binary format, and the meaning of their fields.

Headers:

Request Header => api_key api_version correlation_id client_id 
  api_key => INT16
  api_version => INT16
  correlation_id => INT32
  client_id => NULLABLE_STRING

Response Header => correlation_id 
  correlation_id => INT32

Produce API (Key: 0):

Produce Request (Version: 0) => acks timeout [topic_data] 
  acks => INT16
  timeout => INT32
  topic_data => topic [data] 
    topic => STRING
    data => partition record_set 
      partition => INT32
      record_set => BYTES

Produce Request (Version: 1) => acks timeout [topic_data] 
  acks => INT16
  timeout => INT32
  topic_data => topic [data] 
    topic => STRING
    data => partition record_set 
      partition => INT32
      record_set => BYTES

Produce Response (Version: 0) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code base_offset 
      partition => INT32
      error_code => INT16
      base_offset => INT64

Produce Response (Version: 1) => [responses] throttle_time_ms 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code base_offset 
      partition => INT32
      error_code => INT16
      base_offset => INT64
  throttle_time_ms => INT32

Fetch API (Key: 1):

Fetch Request (Version: 0) => replica_id max_wait_time min_bytes [topics] 
  replica_id => INT32
  max_wait_time => INT32
  min_bytes => INT32
  topics => topic [partitions] 
    topic => STRING
    partitions => partition fetch_offset max_bytes 
      partition => INT32
      fetch_offset => INT64
      max_bytes => INT32

Fetch Request (Version: 1) => replica_id max_wait_time min_bytes [topics] 
  replica_id => INT32
  max_wait_time => INT32
  min_bytes => INT32
  topics => topic [partitions] 
    topic => STRING
    partitions => partition fetch_offset max_bytes 
      partition => INT32
      fetch_offset => INT64
      max_bytes => INT32

Fetch Response (Version: 0) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code high_watermark record_set 
      partition => INT32
      error_code => INT16
      high_watermark => INT64
      record_set => BYTES

Fetch Response (Version: 1) => throttle_time_ms [responses] 
  throttle_time_ms => INT32
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code high_watermark record_set 
      partition => INT32
      error_code => INT16
      high_watermark => INT64
      record_set => BYTES

Offsets API (Key: 2):

Offsets Request (Version: 0) => replica_id [topics] 
  replica_id => INT32
  topics => topic [partitions] 
    topic => STRING
    partitions => partition timestamp max_num_offsets 
      partition => INT32
      timestamp => INT64
      max_num_offsets => INT32

Offsets Response (Version: 0) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code [offsets] 
      partition => INT32
      error_code => INT16

Metadata API (Key: 3):

Metadata Request (Version: 0) => [topics] 

Metadata Response (Version: 0) => [brokers] [topic_metadata] 
  brokers => node_id host port 
    node_id => INT32
    host => STRING
    port => INT32
  topic_metadata => topic_error_code topic [partition_metadata] 
    topic_error_code => INT16
    topic => STRING
    partition_metadata => partition_error_code partition_id leader [replicas] [isr] 
      partition_error_code => INT16
      partition_id => INT32
      leader => INT32

LeaderAndIsr API (Key: 4):

LeaderAndIsr Request (Version: 0) => controller_id controller_epoch [partition_states] [live_leaders] 
  controller_id => INT32
  controller_epoch => INT32
  partition_states => topic partition controller_epoch leader leader_epoch [isr] zk_version [replicas] 
    topic => STRING
    partition => INT32
    controller_epoch => INT32
    leader => INT32
    leader_epoch => INT32
    zk_version => INT32
  live_leaders => id host port 
    id => INT32
    host => STRING
    port => INT32

LeaderAndIsr Response (Version: 0) => error_code [partitions] 
  error_code => INT16
  partitions => topic partition error_code 
    topic => STRING
    partition => INT32
    error_code => INT16

StopReplica API (Key: 5):

StopReplica Request (Version: 0) => controller_id controller_epoch delete_partitions [partitions] 
  controller_id => INT32
  controller_epoch => INT32
  delete_partitions => INT8
  partitions => topic partition 
    topic => STRING
    partition => INT32

StopReplica Response (Version: 0) => error_code [partitions] 
  error_code => INT16
  partitions => topic partition error_code 
    topic => STRING
    partition => INT32
    error_code => INT16

UpdateMetadata API (Key: 6):

UpdateMetadata Request (Version: 0) => controller_id controller_epoch [partition_states] [live_brokers] 
  controller_id => INT32
  controller_epoch => INT32
  partition_states => topic partition controller_epoch leader leader_epoch [isr] zk_version [replicas] 
    topic => STRING
    partition => INT32
    controller_epoch => INT32
    leader => INT32
    leader_epoch => INT32
    zk_version => INT32
  live_brokers => id host port 
    id => INT32
    host => STRING
    port => INT32

UpdateMetadata Request (Version: 1) => controller_id controller_epoch [partition_states] [live_brokers] 
  controller_id => INT32
  controller_epoch => INT32
  partition_states => topic partition controller_epoch leader leader_epoch [isr] zk_version [replicas] 
    topic => STRING
    partition => INT32
    controller_epoch => INT32
    leader => INT32
    leader_epoch => INT32
    zk_version => INT32
  live_brokers => id [end_points] 
    id => INT32
    end_points => port host security_protocol_type 
      port => INT32
      host => STRING
      security_protocol_type => INT16

UpdateMetadata Response (Version: 0) => error_code 
  error_code => INT16

UpdateMetadata Response (Version: 1) => error_code 
  error_code => INT16

ControlledShutdown API (Key: 7):

ControlledShutdown Request (Version: 1) => broker_id 
  broker_id => INT32

ControlledShutdown Response (Version: 1) => error_code [partitions_remaining] 
  error_code => INT16
  partitions_remaining => topic partition 
    topic => STRING
    partition => INT32

OffsetCommit API (Key: 8):

OffsetCommit Request (Version: 0) => group_id [topics] 
  group_id => STRING
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset metadata 
      partition => INT32
      offset => INT64
      metadata => STRING

OffsetCommit Request (Version: 1) => group_id group_generation_id member_id [topics] 
  group_id => STRING
  group_generation_id => INT32
  member_id => STRING
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset timestamp metadata 
      partition => INT32
      offset => INT64
      timestamp => INT64
      metadata => STRING

OffsetCommit Request (Version: 2) => group_id group_generation_id member_id retention_time [topics] 
  group_id => STRING
  group_generation_id => INT32
  member_id => STRING
  retention_time => INT64
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset metadata 
      partition => INT32
      offset => INT64
      metadata => STRING

OffsetCommit Response (Version: 0) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code 
      partition => INT32
      error_code => INT16

OffsetCommit Response (Version: 1) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code 
      partition => INT32
      error_code => INT16

OffsetCommit Response (Version: 2) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code 
      partition => INT32
      error_code => INT16

OffsetFetch API (Key: 9):

OffsetFetch Request (Version: 0) => group_id [topics] 
  group_id => STRING
  topics => topic [partitions] 
    topic => STRING
    partitions => partition 
      partition => INT32

OffsetFetch Request (Version: 1) => group_id [topics] 
  group_id => STRING
  topics => topic [partitions] 
    topic => STRING
    partitions => partition 
      partition => INT32

OffsetFetch Response (Version: 0) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition offset metadata error_code 
      partition => INT32
      offset => INT64
      metadata => STRING
      error_code => INT16

OffsetFetch Response (Version: 1) => [responses] 
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition offset metadata error_code 
      partition => INT32
      offset => INT64
      metadata => STRING
      error_code => INT16

GroupCoordinator API (Key: 10):

GroupCoordinator Request (Version: 0) => group_id 
  group_id => STRING

GroupCoordinator Response (Version: 0) => error_code coordinator 
  error_code => INT16
  coordinator => node_id host port 
    node_id => INT32
    host => STRING
    port => INT32

JoinGroup API (Key: 11):

JoinGroup Request (Version: 0) => group_id session_timeout member_id protocol_type [group_protocols] 
  group_id => STRING
  session_timeout => INT32
  member_id => STRING
  protocol_type => STRING
  group_protocols => protocol_name protocol_metadata 
    protocol_name => STRING
    protocol_metadata => BYTES

JoinGroup Response (Version: 0) => error_code generation_id group_protocol leader_id member_id [members] 
  error_code => INT16
  generation_id => INT32
  group_protocol => STRING
  leader_id => STRING
  member_id => STRING
  members => member_id member_metadata 
    member_id => STRING
    member_metadata => BYTES

Heartbeat API (Key: 12):

Heartbeat Request (Version: 0) => group_id group_generation_id member_id 
  group_id => STRING
  group_generation_id => INT32
  member_id => STRING

Heartbeat Response (Version: 0) => error_code 
  error_code => INT16

LeaveGroup API (Key: 13):

LeaveGroup Request (Version: 0) => group_id member_id 
  group_id => STRING
  member_id => STRING

LeaveGroup Response (Version: 0) => error_code 
  error_code => INT16

SyncGroup API (Key: 14):

SyncGroup Request (Version: 0) => group_id generation_id member_id [group_assignment] 
  group_id => STRING
  generation_id => INT32
  member_id => STRING
  group_assignment => member_id member_assignment 
    member_id => STRING
    member_assignment => BYTES

SyncGroup Response (Version: 0) => error_code member_assignment 
  error_code => INT16
  member_assignment => BYTES

DescribeGroups API (Key: 15):

DescribeGroups Request (Version: 0) => [group_ids] 

DescribeGroups Response (Version: 0) => [groups] 
  groups => error_code group_id state protocol_type protocol [members] 
    error_code => INT16
    group_id => STRING
    state => STRING
    protocol_type => STRING
    protocol => STRING
    members => member_id client_id client_host member_metadata member_assignment 
      member_id => STRING
      client_id => STRING
      client_host => STRING
      member_metadata => BYTES
      member_assignment => BYTES

ListGroups API (Key: 16):

ListGroups Request (Version: 0) => 

ListGroups Response (Version: 0) => error_code [groups] 
  error_code => INT16
  groups => group_id protocol_type 
    group_id => STRING
    protocol_type => STRING

Some Common Philosophical Questions

Some people have asked why we don't use HTTP. There are a number of reasons, the best is that client implementors can make use of some of the more advanced TCP features--the ability to multiplex requests, the ability to simultaneously poll many connections, etc. We have also found HTTP libraries in many languages to be surprisingly shabby.

Others have asked if maybe we shouldn't support many different protocols. Prior experience with this was that it makes it very hard to add and test new features if they have to be ported across many protocol implementations. Our feeling is that most users don't really see multiple protocols as a feature, they just want a good reliable client in the language of their choice.

Another question is why we don't adopt XMPP, STOMP, AMQP or an existing protocol. The answer to this varies by protocol, but in general the problem is that the protocol does determine large parts of the implementation and we couldn't do what we are doing if we didn't have control over the protocol. Our belief is that it is possible to do better than existing messaging systems have in providing a truly distributed messaging system, and to do this we need to build something that works differently.

A final question is why we don't use a system like Protocol Buffers or Thrift to define our request messages. These packages excel at helping you to managing lots and lots of serialized messages. However we have only a few messages. Support across languages is somewhat spotty (depending on the package). Finally the mapping between binary log format and wire protocol is something we manage somewhat carefully and this would not be possible with these systems. Finally we prefer the style of versioning APIs explicitly and checking this to inferring new values as nulls as it allows more nuanced control of compatibility.

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